Low carbon-high manganese steel and manufacturing process thereof

ABSTRACT

A low carbon-high manganese steel sheet and process for manufacturing the sheet is provided. The process includes soaking a steel slab with a desired chemical composition within a temperature range of 1200-1350° C., followed by hot rolling of the slab into hot strip. The cold rolled sheet is continuously annealed within a temperature range of 730-850° C. and temper rolled between 1.0-2.0%. The temper rolled sheet has a yield strength greater than 280 megapascals (MPa), a tensile strength greater than 400 MPa, an elongation to fracture greater than 30%, an n-value greater than 0.15, and a bakehard index between 15-35 MPa.

CROSS-REFERENCE TO RELATED APPLICATIONS

The instant application claims priority of U.S. Provisional Application No. 61/782,417 filed on Mar. 14, 2013, the contents of which are incorporated herein in their entirety by reference.

BACKGROUND OF THE INVENTION

With ever increasing demands for improved vehicle fuel economy, optimized designs of outer vehicle body panels such as bumper parts must meet several performance criteria including stiffness; resistance to oil canning which is also known as critical buckling load; and dent resistance. In addition, thin gauge paint systems recently introduced for automotive exposed parts require an extremely smooth substrate surface after forming for a good paint appearance. Steels have typically been used for such automotive parts and can provide superior surface quality, excellent formability and paintability compared to other materials.

Dent resistant steels are typically produced using vacuum degas sed ultra low carbon (ULC) or regular low-carbon steel compositions. However, ULC steel grades are limited to a yield strength of 280 megapascals (MPa) and involve expensive melting and casting processes such as vacuum treatment. Also, ULC bake-hardenable grades with reduced residual carbon in solution deliver unsuitable surface qualities after painting via the thin gauge paint systems. On the other hand, low-carbon peritectic steel compositions with 0.09-0.20 weight percent (wt %) carbon (C) (see FIG. 1) are susceptible to casting defects developed from the peritectic phase transition which causes heterogeneous shrinkage and significant stress during solidification. Such shrinkage and stress tends to result in facial defects, such as cracking, lamination, and oscillation marks that cause difficulty in continuously casting these types of steels. Therefore, with these defects occurring at increased rates, such steel grades are less suitable for superior surface quality requirements.

Low-carbon non-peritectic compositions with 0.020-0.045 wt % C do not suffer from the peritectic phase transition; however do have unstabilized residual carbon in solid solution. Therefore, uncontrolled aging that can cause coil breaks and deteriorate the surface quality of sheet material make these grades unsuitable for exposed applications.

Given the above, a cost-effective dent resistant steel grade that is not prone to uncontrolled aging, has a non-peritectic alloy composition, and does not require expensive vacuum or scarfing treatments would be desirable.

SUMMARY OF THE INVENTION

A process for making a low carbon-high manganese steel sheet is provided. The process includes providing a steel slab having a chemical composition in weight percent (wt %) within a range of 0.04-0.10 carbon (C), 0.80-1.65 manganese (Mn), 0.5 maximum (max) silicon (Si), 0.40 max chromium (Cr), 0.10 max niobium (Nb), 0.03 max titanium (Ti), 0.003 max vanadium (V), 0.20 max molybdenum (Mo), 0.10 max nickel (Ni), 0.015 max sulfur (S), 0.05 max phosphorus (P), 0.012 max nitrogen (N), 0.003 max boron (B), and 0.015-0.065 aluminum (Al), with the balance being iron (Fe) and incidental impurities.

The steel slab is soaked within a temperature range of 1200-1350° C. and then hot rolled to produce a transfer bar using a roughing treatment. The transfer bar is hot rolled using a finishing treatment in order to produce hot rolled strip. The finishing treatment has an entry temperature between 950-1150° C. and an exit temperature between 800-950° C. Furthermore, the hot rolled strip is coiled within a temperature range of 550-730° C.

The hot rolled strip is cold rolled in order to produce cold rolled sheet, the cold rolled sheet having a reduction in thickness ranging between 50-80% compared to the thickness of the hot rolled strip. The cold rolled sheet is continuously annealed within a temperature range of 730-850° C., followed by a 1.0-2.0% skin pass or temper roll of the annealed cold rolled sheet. The continuously annealed sheet can be cooled from the annealing temperature using a Jet cooling treatment that employs an average cooling rate between 4-100° C./s. In addition, the continuously annealed sheet can be subjected to a second cooling treatment that uses an average cooling rate between 3-20° C./s. The continuously annealed sheet is cooled to a temperature equal to or less than 200° C. before it is temper rolled. Finally, the temper rolled sheet has a thickness between 0.30-2.5 millimeters (mm).

Regarding mechanical properties, the temper rolled sheet has a 0.2% yield strength greater than 280 megapascals (MPa), a tensile strength greater than 400 MPa, an elongation to fracture greater than 30% and a bakehard index (BH2) between 15-35 MPa. The temper rolled sheet has an n-value greater than 0.15.

A dent resistant low carbon-high manganese steel sheet is also provided, the steel sheet having a chemical composition within the range disclosed above and having a completely recovered and recrystallized microstructure. The sheet also has a yield strength greater than 280 MPa, a tensile strength greater than 400 MPa, an elongation to fracture greater than 30% and a bakehard index (BH2) between 15-35 MPa. In some instances, the cold rolled sheet has been temper rolled between 1.0-2.0% and does not exhibit a yield point elongation known to those skilled in the art for typical low carbon steels. Also, the cold rolled steel sheet can have an n-value greater than 0.15.

In some instances, the cold rolled steel sheet has at least 0.015 wt % of free or uncombined Al. Finally, the cold rolled steel sheet has a thickness between 0.30-2.5 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical illustration of manganese content versus carbon content for steels in which the peritectic phase transition is present;

FIG. 2 is a graphical plot of yield strength versus position or location on a coil—head (H), middle (M), tail (T)—for alloys according to an embodiment of the present invention;

FIG. 3 is a graphical plot for tensile strength versus position or location on a coil—head (H), middle (M), tail (T)—for alloys according to an embodiment of the present invention;

FIG. 4 is a graphical plot for elongation to fracture versus position or location on a coil—head (H), middle (M), tail (T)—for alloys according to an embodiment of the present invention;

FIG. 5 is a graphical plot of n-value versus position—head (H), tail (T)—for alloys according to an embodiment of the present invention;

FIG. 6 is a graphical plot of BH2 for standard bake hardenable alloys versus alloys according to an embodiment of the present invention;

FIG. 7 is a graphical plot of yield point elongation versus skin pass degree for alloys according to an embodiment of the present invention;

FIG. 8A is an optical micrograph of the microstructure of a steel coil processed according to an embodiment of the present invention, etched with 2% nital and taken at 100× in the longitudinal direction of the coil;

FIG. 8B is an optical micrograph of the microstructure of a steel coil processed according to an embodiment of the present invention, etched with 2% nital and taken at 100× in the transverse direction of the coil;

FIG. 9A is an optical micrograph of the microstructure of a steel coil processed according to an embodiment of the present invention, etched with 2% nital and taken at 500× in the longitudinal direction of the coil;

FIG. 9B is an optical micrograph of the microstructure of a steel coil processed according to an embodiment of the present invention, etched with 2% nital and taken at 500× in the transverse direction of the coil;

FIG. 10A is an optical micrograph of the microstructure of a steel coil processed according to an embodiment of the present invention, etched with 2% nital and taken at 1000× in the longitudinal direction of the coil; and

FIG. 10B is an optical micrograph of the microstructure of a steel coil processed according to an embodiment of the present invention, etched with 2% nital and taken at 1000× in the transverse direction of the coil.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a steel and a process for making a steel that exhibits a 0.2% yield strength greater than 280 MPa, a tensile strength greater than 400 MPa, an elongation to fracture of greater than 30%, a bakehard index between 15-35 MPa and optionally an n-value greater than 0.15. The steel with such properties is a cold rolled continuously annealed steel sheet made from an aluminum-killed low alloy steel having a chemical composition in weight percent (wt %) within a range of 0.04-0.10 C, 0.80-1.65 Mn, 0.5 max Si, 0.40max Cr, 0.10 max Nb, 0.03 max Ti, 0.003 max V, 0.20 max Mo, 0.10 max Ni, 0.015 max S, 0.05 max P, 0.012 max N, 0.003 max B, and 0.015-0.065 Al, with the balance being iron (Fe) and incidental impurities known to those skilled in the art of steel production and manufacturing.

Bake hardening is a “controlled aging” phenomenon related to the presence of carbon/or nitrogen in solid solution in the steel. The BH2 parameter is used to evaluate the resulting increase in dent resistance. Calculated amounts of alloying elements such as Mn, V, Ti, Al, etc. are added to control the free carbon/or nitrogen in steel that can contribute to BH2.

It should be appreciated that Mn decreases the C activity and thereby decreases the aging contribution of solute carbon. Mn interacts with interstitial solute atoms (C and N) and forms interstitial-substitutional solute pairs. These Mn-C or/and Mn-N pairs reduces the mobility of the interstitial solutes to interact with dislocations. So, it is expected that addition of Mn leads to a reduction in strain aging. The Mn also aids in non-aging because of its dual role as a diluent and distributor of nitride forming reactive elements. In addition, Mn is capable of forming manganese sulfide (MnS) by reacting with S, which is known to be beneficial to the machining of the steel. At the same time, Mn counters the brittleness due to S content in the steel and also provides an improved surface finish for the material.

The Ti content of the steel is chosen to be equal to or slightly above a stoichiometric quantity necessary to bind with N within the material. Aluminum is added to substantially reduce the amount of oxygen present in the casting of a slab of the steel. The amount of Al added is carefully controlled such that oxygen is substantially eliminated and a small amount such as at least 0.015 wt % of uncombined or free Al remains. In addition, uncombined Al interacts with free nitrogen to form AlN and the formation of AlN reduces the propensity of nitrogen contribution to aging.

Hot rolled steel strip with a composition within the above-identified chemistry range and properties mentioned above is provided by soaking a slab of steel with the desired chemistry at temperatures between 1200-1350° C., followed by rolling of the slab in a roughing mill as known to those skilled in the art in order to produce a transfer bar for finish rolling. Such finish rolling typically occurs between 800-950° C. and provides a hot band with a thickness between 1.50-6.50 millimeters having a uniform grain size, bulk, and surface finish. For example, in some instances the finishing treatment has an entry temperature between 950-1150° C. and an exit temperature between 800-950° C. Also, coiling temperatures for the hot band are between 550-730° C.

The hot rolled sheet is then uncoiled and subjected to a cold rolling treatment with a reduction in sheet thickness ranging between 50-80%. After cold rolling, the cold rolled sheet is annealed in a continuous annealing line (CAL) within the temperature range of 730 to 850° C. and then cooled at average cooling rates between 4° C./s and 100° C./s. The CAL speed during the annealing process is between 30 and 200 meters per minute (m/min) and time in the soaking zone of the furnace is between 0.50-5.0 min. After rapid cooling, the cold rolled and annealed steel sheet is subjected to slower cooling rates between 3° C./s and 20° C./s until the material is cooled to approximately 200° C. or lower. The cold rolled sheet is then subjected to skin pass or temper rolling between 1.0 and 2.0%. In this manner, a cold rolled sheet having a thickness between 0.30 and 2.5 mm that has been annealed and has a microstructure that is completely recovered and recrystallized is provided. Furthermore, it should be appreciated that the skin pass or temper rolling of 1.0-2% reduces or eliminates the pronounced yield point elongation such that coil break appearance is avoided.

In order to provide a specific example of the invention and yet not limit its scope in any way, an example of a composition and a process according to an embodiment of the present invention is provided below.

EXAMPLE

A steel slab having a thickness of approximately 250 mm and a chemical composition in wt % of 0.05-0.07 C, 1.20-1.35 Mn, 0.03 Si, 0.013-023 Ti, 0.008 V, 0.05 Mo, 0.05 Cu, 0.01 S, 0.015 P, 0.003-0.007 N, and 0.02-0.04 Al, with the balance being Fe and incidental impurities was soaked at 1285° C. and then subjected to a roughing treatment. The roughing treatment provided a transfer bar having a thickness of 48 mm which was then subjected to a finishing treatment. The temperature at the entry of the finishing treatment for the slab was 1055° C. and the temperature at the exit of the finishing treatment was 870° C. The hot strip produced by the roughing and finishing treatment had a thickness between 3.65 and 3.68 mm which was coiled at 640° C.

The hot strip coil was cold rolled 72% and annealed at 800° C. for 55 seconds with a CAL speed of 120 m/min and a skin pass degree of 1.7%. The cold rolled and annealed sheet had a thickness of 1.00 mm. The finished material sheet was subjected to mechanical testing and exhibited a yield strength of 334 MPa, a tensile strength of 445 MPa, a percent elongation to failure of 36.0%, and an n-value of 0.17. The finished material was subjected to bake hardening testing and exhibited a BH2 of 30 MPa. For the purposes of the present invention, the n-value is defined by the expression of the form σ=Kε where for an induced strain c, the corresponding stress σ is the new yield strength of the material caused by the degree of cold working that has induced the strain ε. As such, and not being bound by theory, the greater the value of n for a material, the greater the degree of work hardening the material exhibits upon cold forming and thus giving a measure of increased global formability. For the purposes of the present invention, BH2 is given by BH2=LYS-2% PYS, in which LYS is the lower yield stress, and PYS is the yield stress measured after heat treatment at 170° C. for 20 minutes after an initial 2% plastic pre-strain.

In addition to the above, mechanical property testing was conducted at different locations within coils having a chemical composition within the range provided above and processed as disclosed herein. Results for the testing are shown in FIGS. 2-5, and as shown in the figures, the yield strength, tensile strength, elongation, and n-value were measured from samples taken from the head (H), middle (M), and tail (T) from a number of coils. As shown by the data, the tested samples from the various locations of the coils provided properties that met or exceeded the requirements of the materials discussed above, i.e. a yield strength greater than 280 MPa, a tensile strength greater than 400 MPa, a percent elongation greater than 30%, and an n-value greater than 0.15.

With reference to FIG. 6, BH2 values for standard bake heardenbale alloys versus alloys according to an embodiment of the present invention are shown. As shown in the figure, typical bake hardenable grades provide a BH2 between 35-70 MPa with an average around 45 MPa. In contrast, the inventive alloys and/or processing disclosed herein provide a BH2 between 15-35 MPa with an average around 30 MPa. As such, the alloys and processing provide a low BH index that provides good control over aging and yet allows bake hardening of a formed component.

FIG. 7 provides data for yield point elongation as a function of skin pass degree. The data in this figure clearly demonstrates that the skin pass degree or temper rolling of greater than 1% eliminates the yield point for the steels tested.

Regarding microstructures of the coiled steel, FIGS. 8A and 8B show optical micrographs of steel coil microstructure processed according to an embodiment of the present invention, etched with 2% nital and taken at 100× in the longitudinal direction and transverse direction, respectively, of the coil. Similarly, FIGS. 9A and 9B show optical micrographs of the steel coil microstructure taken at 500× in the longitudinal direction and transverse direction, respectively. Finally, FIGS. 10A and 10B show optical micrographs taken at 1000× in the longitudinal direction and transverse direction, respectively. As illustrated by the micrographs, the steel coil has a completely recovered and recrystallized microstructure.

The properties shown in the figures and disclosed herein are appreciated to be unknown in the prior art for such low carbon-high manganese steels and thus exhibit unexpected results. In addition, the steels disclosed herein can be made without expensive vacuum or scarfing treatments and do not develop casting defects from a peritectic phase transition upon cooling/solidification. Defects such as heterogeneous shrinkage that add significant stress during the solidification process and thus result in facial defects such as cracking, lamination, and oscillation marks are eliminated.

The steels and the processes disclosed herein also provide for a lower occurrence of long wave roughness compared to standard bake hardening (BH) steels. As such, the inventive steels provide a superior surface finish after being formed but before being painted, when compared to standard BH steels, and thus allow for newer thinner paint systems being utilized in the automotive industry to be employed. In addition, standard BH hardening steels fall outside the scope of the present invention and typically have less than 0.65 wt % Mn. The steels disclosed herein also do not experience uncontrolled aging and thus prevent coil breaks that deteriorate the surface quality of the material and make it suitable for environmentally exposed applications.

In view of the teaching presented herein, it is to be understood that numerous modifications and variations of the present invention will be readily apparent to those of the skill in the art. The foregoing is illustrative of specific embodiments and/or examples of the invention, but is not meant to be a limitation upon the practice thereof. As such, the scope of the invention is covered by the claims and all equivalents thereof. 

We claim:
 1. A process for making a low carbon-high manganese steel sheet, the process comprising: providing a steel slab having a chemical composition within a range in weight percent of 0.04-0.10 C, 0.80-1.65 Mn, 0.5 max Si, 0.40 max Cr, 0.10 max Nb, 0.03 max Ti, 0.003 max V, 0.20 max Mo, 0.10 max Ni, 0.015 max S, 0.05 max P, 0.012 max N, 0.003 max B, and 0.015-0.065 Al, with the balance being Fe and incidental impurities; soaking the steel slab within a temperature range of 1200-1350° C.; hot rolling the soaked steel slab using a roughing treatment and producing a transfer bar; hot rolling the transfer bar using a finishing treatment and producing hot rolled strip; cold rolling the hot rolled strip and producing cold rolled sheet with a reduction in thickness ranging between 50-80%; continuous annealing the cold rolled sheet within a temperature range of 730-850° C.; and temper rolling the annealed cold rolled sheet between 1.0-2.0%, the temper rolled sheet having a yield strength greater than 280 MPa, a tensile strength greater than 400 MPa and an elongation to fracture greater than 30%.
 2. The process of claim 1, wherein the temper rolled sheet has an n-value greater than 0.15.
 3. The process of claim 2, wherein the finishing treatment has an entry temperature between 950-1150° C. and an exit temperature between 800-950° C.
 4. The process of claim 3, further including coiling the hot rolled strip within a temperature range of 550-730° C.
 5. The process of claim 4, further including cooling the continuous annealed sheet during a first cooling treatment using an average cooling rate between 4-100° C./s.
 6. The process of claim 5, further including cooling the continuous annealed sheet during a second cooling treatment using an average cooling rate between 3-20° C./s.
 7. The process of claim 6, wherein the continuous annealed sheet is cooled to a temperature equal to or less than 200° C. before being temper rolled.
 8. The process of claim 7, wherein the temper rolled sheet has a thickness between 0.30-2.5 mm.
 9. The process of claim 7, further including bake hardening the continuous annealed sheet, the continuous annealed sheet having a BH2 of between 15-35 MPa.
 10. A process for making a dent resistant low carbon-high manganese steel sheet, the process comprising: providing a steel slab having a chemical composition with the range in weight percent of 0.04-0.10 C, 0.80-1.65 Mn, 0.5 max Si, 0.40 max Cr, 0.10 max Nb, 0.03 max Ti, 0.003 max V, 0.20 max Mo, 0.10 max Ni, 0.015 max S, 0.05 max P, 0.012 max N, 0.003 max B, and 0.015-0.065 Al, with the balance being Fe and incidental impurities; soaking the steel slab within a temperature of 1200-1350° C.; hot rolling the soaked steel slab using a roughing treatment and producing a transfer bar; hot rolling the transfer bar using a finishing treatment and producing hot rolled strip, the finishing treatment having an entry temperature between 950-1150° C. and an exit temperature between 800-950° C.; coiling the hot rolled strip within a temperature range of 550-730° C.; cold rolling the hot rolled strip and producing cold rolled sheet with a reduction in thickness between 50-80%; continuous annealing the cold rolled sheet within a temperature range of 730-850° C.; and temper rolling the annealed cold rolled sheet between 1.0-2.0%, the temper rolled sheet having a yield strength greater than 280 MPa, a tensile strength greater than 400 MPa, an elongation to fracture greater than 30% and an n-value greater than 0.15.
 11. The process of claim 10, wherein the continuous annealed cold rolled sheet is cooled to a temperature equal to or less than 200° C. using a first cooling treatment having an average cooling rate between 4-100° C./s and a second cooling treatment having an average cooling rate between 3-20° C./s.
 12. The process of claim 11, wherein the temper rolled sheet has a thickness between 0.30-2.5 mm.
 13. A dent resistant low carbon-high manganese steel sheet comprising: a cold rolled steel sheet having a thickness between 0.20-2.5 mm and a chemical composition within a range in weight percent of 0.04-0.10 C, 0.80-1.65 Mn, 0.5 max Si, 0.40 max Cr, 0.10 max Nb, 0.03 max Ti, 0.003 max V, 0.20 max Mo, 0.10 max Ni, 0.015 max S, 0.05 max P, 0.012 max N, 0.003 max B, and 0.015-0.065 Al, with the balance being Fe and incidental impurities; a completely recovered and recrystallized microstructure; and a yield strength greater than 280 MPa, a tensile strength greater than 400 MPa and an elongation to fracture greater than 30%.
 14. The steel sheet of claim 13, wherein said cold rolled steel sheet has been temper rolled between 1.0-2.0% and has no yield point elongation.
 15. The steel sheet of claim 14, wherein said cold rolled steel sheet has an n-value greater than 0.15.
 16. The steel sheet of claim 15, wherein said cold rolled steel sheet has at least 0.015 wt % free Al.
 17. The steel sheet of claim 16, wherein said cold rolled steel sheet has a thickness between 0.30-2.5 mm.
 18. The steel sheet of claim 17, wherein said cold rolled steel has a BH2 of between 15-35 MPa. 